A known circuit arrangement is disclosed, for example, in the Laid-Open Patent Specification EP 0 753 987 A1. This circuit arrangement has a half-bridge inverter with a switch-off device which switches off the half-bridge inverter in the case of an anomalous operating state--for example in the case of a lamp which refuses to start or is defective. The switch-off device has a field effect transistor whose drain-source path is arranged in the control circuit of a half-bridge inverter transistor and switches the control circuit between a low-resistance and a high-resistance state. Upon the occurrence of an anomalous operating state, the switch off is performed synchronously with the blocking phase of that half-bridge inverter transistor in whose control circuit the field effect transistor is arranged. The switch-off device of this circuit arrangement certainly switches the half-bridge inverter off reliably in the case of a lamp which refuses to start, but it reacts in general too insensitively to the occurrence of the so-called rectifying effect of the discharge lamp, which will be explained in more detail below.
A possible cause of failure of discharge lamps, in particular of low-pressure discharge lamps, is occasioned by a reduction over the lifetime of the lamp in the ability of the lamp electrodes to emit. Since the loss of the ability to emit generally proceeds with varying intensity in the two lamp electrodes over the lifetime of the lamp, at the end of the lifetime of a discharge lamp operated with alternating current a preferred direction has been formed for the discharge current through the discharge lamp. The discharge lamp develops a current-rectifying effect in this case. This effect is designated as a rectifying effect of the discharge lamp. Owing to the occurrence of the rectifying effect in the discharge lamp, the lamp electrode incapable of emission is heated extremely, with the result that impermissibly high temperatures can occur which can even cause melting of the lamp bulb glass.
Moreover, in the case of discharge lamps which are operated on a half-bridge inverter, the rectifying effect of the discharge lamp causes a substantial deviation in the voltage drop across the coupling capacitor or the coupling capacitors from the normal value, which is usually half as large as the value of the input voltage of the half-bridge inverter. In the case of self-oscillating half-bridge inverters, this deviation in the voltage drop across the coupling capacitor or the coupling capacitors leads to stopping the oscillation of the half-bridge inverter, because the supply voltage of one of the two half-bridge branches is in this case too low to maintain the feedback. However, immediately after being interrupted the oscillation of the half-bridge inverter is set going again by the starting circuit of the half-bridge inverter if the switch-off device is not triggered. As a result, the discharge lamp affected by the rectifying effect is not reliably switched off, but flickers instead.